Jonathan Simms, 20, from Belfast, was a talented youth footballer when he was diagnosed with variant Creutzfeldt-Jakob Disease (vCJD). As his health rapidly declined his family went to court for him to be allowed to take the drug pentosan polysulphate.

In Britain 147 people have died from the rare brain disease. Jonathan Simms is the only patient known to have survived three years while suffering from it. Usually patients diagnosed with it survive about 14 months.

"The general consensus. . . is that Jonathan Simms is no longer terminally ill," his father, Don, told BBC Radio Ulster following a medical evaluation of his son.

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Dr Stephen Dealler, consultant in mico-biology at Lancaster Royal Infirmary and one of a team of CJD experts involved in Mr Simms' case, confirmed last night that the teenager's condition had become stable.

"Everyone was shocked at how he responded. We had expected him to die in April but at a meeting to discuss his progress several months ago people realised that he had not gone downhill. In fact, there was some evidence that the disease had disappeared, although nobody is able to say he will get better. His condition certainly seems to have stabilised."

Dr Dealler said Mr Simms was the first patient to receive the treatment and had so far shown the most progress.

"So far about 12 people have been treated," he said. "One died despite treatment, some have continued without improvement but the condition of about six has stabilised."

After a legal battle with the NHS during which Mr Simms's condition deteriorated to the point where he needed constant care, his father won permission in December 2002 to start the experimental treatment.

Mr Simms has since undergone two years of having pentosan polysulphate injected directly into his brain.

Because the drug, which is used in America to treat cystitis, has anticoagulant blood thinner and anti-inflammatory properties it was thought to carry the risk of side effects.

"If we had been shown and, it is still the case, that pentosan was causing Jonathan severe pain and stress we would have to take him off that compound, we would have to kiss him and let him go," Mr Simms' father said.